It's evolution. Back in the day the big name shooters who represented not only manufacturers but ammo companies as well all shot SXS's. Remember that's how Arthur DuBray made his living, selling Parker's to competitive shooters as well as the general public.
In 1948 my Grandfather won the Class C championship at the Detroit Gun Club by breaking 200 straight from 16 yards. I have the trophy. According to my Mom his favorite trap gun was a Parker.

I will be the first to admit I don't shoot my small frame Parkers as well as I do a Browning Superposed. In part because I used to be addicted to trap and shot a Broadway for years.
I am planning to shoot a few clays this year with my Parkers and see if I can't improve.

I choose to hunt birds exclusively with Parkers, it just adds to the fun, my game bag doesn't always need to be heavy. Today it's much more about watching the dog work and see him get hot. If I knock one down for him once in awhile he's happy. If I miss them all he still gets in the truck and goes home with me.

Maybe it's evolution out with the old in with the new sort of thing. I don't buy into the single sighting plane mumbo jumbo. Back in the day when the shooting sports both live birds and clay targets were both not only competitive but spectator events the SXS was the gun used and used well by top shooters across the country. Arthur DuBray made his living selling Parkers to shooters who were not only being endorsed by gun company but by ammo companies as well.

I doubt you will see a Beretta on a Grouse Moore across the pond.

In 1948 my Grandfather won the class C championship at the Detroit Gun Club by breaking 200 straight from 16 yards. I have the trophy he won. According to my Mom his favorite trap gun was a Parker.

Rise & Fall of American SxS's
 

An Outdoor Life article on The Rise and Fall of the American Side-by-Side Shotgun, The highs and lows of a classic icon

https://www.outdoorlife.com/rise-and...tlua00sfOpq.03

There are a few nuggets of wisdom in this piece. If you know how to read into them.

No. 1:

"More and more, it appears that the side-by-side shotgun, both American and European, is fated to become a niche item—the Morgan sportscar of the shooting world."

And No. 2:

"...we can never escape one crucial fact: A fine side-by-side is a thing of beauty."

In plain language, the SxS double gun was revived and thrived because it had cachet and appealed to a segment of society that knew what a Morgan was. They also appreciated beauty. Beauty of design, fine craftsmanship... and style.

As these people and their more sophisticated taste leave the marketplace, the magic of great double guns fades from the public's collective conscious.

To be replaced with movie and video game-inspired black guns. Killing machines without soul or grace. Suitable for the mass murder of wildlife. And nothing more.

The perfect reflection of the times in which we live.

The Morgan - Ah yes, one of the few cars with a wood frame, and I think the early ones were three wheelers

I don't think this lack of interest is confined to SXS. Most young people will pick up a synthetic stocked rifle package for under $500 vs a pre64 M70. The nostalgia and panash of the older guns is lost on out youth I'm afraid.

yes there was a three wheel Morgan, the frames were and are steel - the body supports were ash as were many English cars - up to a Rolls
the MG -up to the T series actually had a partial wood frame plus the supports

but the Morgan is a survivor- the newest ones still exude the same class

on topic - i think the SxS has been a niche for a long time - and likely getting to be smaller - since our fathers (for us old guys) came home from WWII

when it was time for me to get my own gun - we went to the shop and I picked out a 20 ga SxS - a Fox B if i remember correctly- my father asked why i wanted one of those old fashion guns and bought me a 16ga pump gun

as for the new shooters- first there are many fewer of them- and secondly - most of us i would bet grew into doubles, the new shooters go with what they can afford and what is advertised to them

The Morgan assembly line

I've been fortunate to have a son (now 23) who blesses every American side by side I own. He loves the look and chit chat of other shooters at the sporting clays range with their semi-autos, pumps, 30"/32" O/U's when he brings out the hammer Parker or Model 21 with 26" barrels and nary missing a shot. Smokes them all. We've both been blessed with a family heritage and tradition of good guns and even better dogs (Llewellins, of course)...a spirit I believe will be lost in the next 50 years if not sooner and that's sad.